API change in hilbert_space_tools

* Rename fundamental_operator_set::n_operators() -> size()
* Rename hilbert_space::dimension() -> size()
* Rename sub_hilbert_space::dimension() -> size()
* Remove fundamental_operator_set::dimension(), because this method
  logically belongs to hilbert_space.
New method overload hilbert_space::get_fock_state()

Used to get a Fock state generated from the vacuum by application
of a number of C^+ operators.